This invention relates generally to a printing unit or processor, such as, an office copier, facsimile or non-impact printer, and more particularly concerns an apparatus for decurling sheet material employed therein.
Generally, electrophotographic printing comprises charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image of the original document being reproduced. This records an electrostatic latent image on the photoconductive member which corresponds to the informational areas contained within the original document being reproduced. This records an electrostatic latent image on the photoconductive member which corresponds to the informational areas contained within the original document. The latent image is developed by bringing a developer material into contact therewith. In this way, a powder image is formed on the photoconductive member which is subsequently transferred to a sheet of support material. The sheet of support material is then heated to permanently affix the powder image thereto.
As the sheet of support material passes through the various processing stations in the electrophotographic printing machine, a curl or bend is frequently induced therein. Occasionally, this curl or bend may be inherent in the sheet of support material due to the method of manufacture thereof. It has been found that this curl is variable from sheet to sheet within the stack of sheets utilized in the printing machine. The curling of the sheet of support material causes problems of handling as the sheet is processed in the printing machine. Sheets delivered in a curled condition have a tendency to have their edges out of registration with the aligning mechanisms employed in the printing machine. In addition, curled sheets tend to produce jams or misfeeds within the printing machine. In the past, this problem has been resolved by utilizing bars, rollers or cylinders which engage the sheet material as it passes through the printing machine. Frequently, belts or soft rollers are used in conjunction with a hard penetrating roll to remove the curl in a sheet. Decurler systems of this type reduce paper curl in the process direction only. But, once the curl in that direction is reduced, the curl in the cross-direction (perpendicular to the process direction) will appear. This phenomenon is due to the isotropic nature of paper curl that the paper fiber structure on one side tends to contract more than the other side in all directions. Originally, if the curl in the process direction is severe, curl-induced stiffness tends to inhibit the curling in the cross-direction. When the process direction curl is removed and the stiffness is reduced, cross curl becomes free to develop.
Various approaches have been devised to improve sheet decurlers to answer some of the above-detailed problems. The following disclosures appear relevant:
U.S. Pat. No. 4,077,519; Patentee: Huber; issued Mar. 7, 1978. PA1 U.S. Pat. No. 4,326,915; Patentee: Mutschler, Jr.; issued Apr. 27, 1982. PA1 U.S. Pat. No. 4,360,356; Patentee: Hall; issued Nov. 23, 1982. PA1 U.S. Pat. No. 4,475,896; Patentee: Bains; issued Oct. 9, 1984. PA1 U.S. Pat. No. 4,977,432; Patentee: Coombs et al.; issued Dec. 11, 1990. PA1 U.S. Pat. No. 5,066,984; Patentee: Coombs; issued Nov. 19, 1991.
The pertinent portions of the foregoing disclosures may be summarized as follows:
Huber describes a curl detector and separator wherein a paper sheet is passed through the nip of a rotating roll and charging roll, and thereafter the sheet is stripped from the rotating roll by a vacuum stripper which allows the sheet to pass between the nip of a subsequent transport roll pair.
Mutschler, Jr. discloses a sheet decurler apparatus wherein a sheet is pressed into contact with a rigid arcuate member in at least two regions. The sheet moves about the arcuate member or rod in a curved path to remove curl in the sheet. The sheet is bent in one direction by a first rod and in another direction by a second rod.
Hall discloses an apparatus for removing curl from continuous web material during its travel through engagement bars that can be adjusted to remove away from image (AI) or toward image (TI) curl.
Bains describes a curling/decurling mechanism that combines a compliant roller with a soft outer layer in a curling roller to form a penetration nip with the compliant roller. Movable parts are employed to control the angle of sheets as they exit from the nip.
Coombs et al. is directed to a decurling device that includes an arcuate concave guide and a feed roll which causes paper to pass between the guide and the feed roll to decurl the paper. A means is included for selectively laterally offsetting paper driven through the device.
Coombs describes a decurling device that includes an arcuate concave guide and a feed roll which causes paper to pass between the guide and the feed roll to decurl the paper. The space between the guide and the roll is greater than the thickness of the paper and the paper is bent in the direction opposite to its curl as it passes through the arched space.
The present invention aims at providing a decurler designed with the capability of reducing cross curl by distributed local bending of sheets.